Particle sensor with low-pressure-drop air flow system

ABSTRACT

Disclosed is a particle sensor having a light beam with a beam long axis and an air flow tube with an inlet end and a particle exit mouth. In an aspect of the invention, the cross-sectional area of the flow passage at the inlet end is greater than the cross-sectional area of the exit mouth. This enlarged area dramatically reduces pressure drop along the tube. The exit mouth is in registry with the light beam and is elongate in a direction substantially parallel to the beam long axis. Thus, particles flowing through the mouth pass through the beam. In another aspect, the invention includes a centrifugal blower which is light in weight and which may be battery powered.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.08/109,007 filed on Aug. 19, 1993 and now abandoned.

FIELD OF THE INVENTION

This invention relates generally to air quality and, more particularly,to instruments for assaying airborne particulates.

BACKGROUND OF THE INVENTION

Particle counters and sensors are used to detect light scattered byparticles entrained in a stream of fluid, e.g., in an air stream. Suchcounters and sensors draw air (with entrained particles) from a room,for example, and flow such air along a tube and through an illuminatedsensor "view volume" to obtain information about the number and size ofsuch particles. Such information results from an analysis of the verysmall amounts of light reflectively "scattered" by the particle as itmoves through the view volume.

Some types of sensors flow such air along an enclosed transparent tube;others "project" the air and accompanying particles at a particular flowrate (often measured in cubic feet per minute) from one tube across anopen space to another tube. In sensors of the latter type, there is notube wall (however transparent such wall may be) to impair lightscattering and collecting. In other words, the particle is brieflyilluminated by a very-small-diameter light beam is it "flies" through anopen space.

Among other uses, particle counters incorporating particle sensors areused to obtain a measure of air quality by providing information as tothe number and size of particles present in some specified volume ofair, e.g., a cubic meter of air. Even work environments which appear tohuman observation to be clean--business offices, manufacturingfacilities and the like--are likely to have substantial numbers ofmicroscopic airborne particles. While such particles are not usuallytroublesome to the human occupants, they can create substantial problemsin certain types of manufacturing operations.

For example, semiconductors and integrated chips are made in what areknown as "clean rooms," the air in which is very well filtered. In fact,clean rooms are usually very slightly pressurized using extremely cleanair so that particle-bearing air from the surrounding environs does notseep in. And the trend in the semiconductor and integrated chipmanufacturing industry is toward progressively smaller products.

A small foreign particle which migrates into such a product duringmanufacture can cause premature failure or outright product rejectioneven before it is shipped to a customer. This continuing"miniaturization" requires corresponding improvements in clean-roomenvironments (and in the related measuring instruments) to help assurethat the number and size of airborne particles are reduced belowpreviously-acceptable levels. While known particle counters and sensorshave been generally acceptable for their intended purpose, certaindisadvantages exist.

A disadvantage of known particle sensors involves the air passage,usually circular, along which air and entrained particles flow. Inparticular, such passage has a very small cross-sectional area. As aresult, the pressure differential between the ends of the passage(sometimes referred to as the "pressure drop" across the passage) isquite high. It is not unusual to encounter a pressure drop in the rangeof 25-70 inches of water at a flow rate of about one cubic foot perminute (CFM). In the field of particle sensors, a pressure drop of 25-70inches of water at that air flow rate is typical.

(Parenthetically, measuring pressure in inches of water is common. Ananalogy is found in older style blood pressure measuring devices whichinclude a column of mercury contained in and visible through a graduatedglass tube. Blood pressure is measured in "millimeters of mercury" andin such older style devices, blood pressure was equal to the columnheight. Blood pressure is still measured in millimeters of mercury but adifferent type of gauge is used to make the measurement.)

Because of the typical pressure drop along the very-small-area air flowpassage, known sensors require a motor-driven positive displacementvacuum pump, usually of the diaphragm or vane type, to create enoughvacuum to overcome such pressure drop. The necessary electric drivemotor and vacuum pump are likely to be relatively heavy. And the motorrequires outlet-sourced power; battery power is not practical because ofthe relatively large amount of power consumed. And because such a sensorrequires an electrical cord and plug, it is not so readily moved fromsite to site, especially remote sites.

While the pressure drop along the air flow passage can be reduced byincreasing the passage cross-sectional area, there is another designconstraint which militates against that approach. To help assureaccuracy in particle sensing and counting, all (or substantially all) ofthe air-entrained particles flowing along the passage must pass throughthe beam of light. Usually, the "flight path" of particles isperpendicular to such beam. However, the light beam is preferablysharply focused and its diameter is very small, e.g., less than about0.1 inch. Therefore, the diameter of the air flow passage cannot beappreciably larger than that of the light beam and still assure thatmost or all of the particles will pass through the light beam and bedetected.

The invention addresses these seemingly intractable difficulties andinconsistent design parameters in a unique and imaginative way.

OBJECTS OF THE INVENTION

It is an object of the invention to provide an improved particle sensorovercoming some of the problems and shortcomings of the prior art.

Another object of the invention is to provide an improved particlesensor in which the air flow passage exhibits exceptionally low pressuredrop at a flow rate of about one CFM.

Another object of the invention is to provide an improved particlesensor in which substantially all of the particles are directed throughthe light beam.

Yet another object of the invention is to provide an improved particlesensor which is lighter in weight than comparable conventional sensors.

Still another object of the invention is to provide an improved particlesensor which is battery powered and highly portable, even to remotesites. How these and other objects are accomplished will become moreapparent from the following descriptions and from the drawing.

SUMMARY OF THE INVENTION

The invention is an improvement in a particle sensor of the type havinga light beam with a beam long axis and also having an air flow tube with(a) an inlet end, and (b) a particle exit mouth. In the improvement, thecross-sectional area of the flow passage at the inlet end is quite largeand is greater than the cross-sectional area of the exit mouth. And theexit mouth is elongate in a direction substantially parallel to the beamlong axis and, preferably, is "race-track" shaped and has first andsecond side edges which are generally parallel to one another.

The flow passage (of relatively large area) dramatically reduces thepressure drop along the tube. And the long, relatively narrow exit mouth(about as wide as the width of the light beam) helps assure thatparticles flowing through the mouth pass through the beam.

More specifically, the air flow tube has an inlet portion and a nozzleportion. The latter has a first inlet section which has a minimumcross-sectional area, i.e., an area less than that of any section alongthe length of the inlet portion. Further, the nozzle portion has a firstnozzle section which has a maximum cross-sectional area, i.e., an areagreater than that of any section along the nozzle portion.

In a highly preferred embodiment, the cross-sectional area of the firstinlet section is no less than the cross-sectional area of the firstnozzle section. Additionally, the inlet portion has an enlarged secondinlet section having a cross-sectional area greater than that of thefirst inlet section. The first inlet section and the first nozzlesection have substantially the same shape, e.g., circular.

In another aspect of the invention, the sensor air flow tube extendsalong a flow axis and the sensor has an air blower (preferably acentrifugal blower) rather than the conventional positive-displacementvacuum pump. The blower has an inlet opening which is substantiallycircular and in concentric registry with the flow axis. In fact, the newsensor has several component parts "stacked" along the flow axis so thatparticle flow from the inlet portion to the blower is in a straightline.

In another aspect, the new sensor makes unique use of a smallcentrifugal blower. Such blowers are used in applications other thanparticle sensors and are employed for their output flow rather than fortheir ability to "pull a vacuum." In the invention, it is the blower airentry port, not the flow-emanating exhaust port which is of interest.

The air entry port is in flow communication with the exit mouth and theblower thereby provides the pressure differential between the inlet endand the exit mouth of the air flow tube.

In fact, substantially all of the air passing through the blower(preferably a centrifugal blower) is drawn from and first through theair flow tube. In that way, the sensor is substantially unaffected byblower-generated contaminants.

The sensor has a sensing cavity and a blower cavity separated from thesensing cavity by a wall. The wall has an aperture through it and theblower is mounted to an annular plate having an opening through it. Theaperture and the opening are also in registry with the flow axis.

In yet other aspects of the invention, the air blower is batterypowered. While battery-powered air blowers per se are known, earlierdesigners in the field of particle sensors have never appreciated how toconstruct an air flow path with sufficiently low pressure drop along itslength that a very low power blower could be used and still provide verygood air flow rate. A preferred blower is of the adjustable speed typefor selecting an air flow rate. Speed adjustment may include closed loopcontrol in connection with a flow meter.

Other detail of the invention are set forth in the following detaileddescription and in the drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a side elevation view of the new sensor. Parts are broken awayand other parts are shown in cross-section.

FIG. 2 is an elevation view of the new sensor taken along the viewingplane 2--2 of FIG. 1. The upper and lower portions and the cover of thesensor are slightly spaced from one another and parts are broken away.

FIG. 3 is a bottom view of the sensor taken along the viewing plane 3--3of FIG. 2. Parts are broken away, other parts are shown in cross-sectionand yet other parts are omitted.

FIG. 4 is an elevation view of the sensor taken along the viewing plane4--4 of FIG. 2. The parts shown as slightly spaced in FIG. 2 are fullyassembled in FIG. 4.

FIG. 5 is a spatial perspective view of aspects of the sensor shown inFIGS. 1-4.

FIG. 6 is a side elevation view in cross-section of the inlet portion ofthe sensor air flow tube.

FIG. 7 is an end elevation view of the inlet portion shown in FIG. 6taken along the viewing plane 7--7 thereof.

FIG. 8 is a side elevation view of the nozzle portion of the sensor airflow tube.

FIG. 9 is an end elevation view of the nozzle portion shown in FIG. 8taken along the viewing plane 9--9 thereof.

FIG. 10 is a top plan view of the nozzle portion shown in FIGS. 8 and 9taken along the viewing plane 10--10 of FIG. 8.

FIG. 11 is a side elevation view in cross-section of the inlet portionof FIGS. 6 and 7 and the nozzle portion of FIGS. 8-10 assembled to oneanother.

FIG. 12 is a cross-section view of the inlet portion of the air flowtube taken along the viewing plane 12-12 of FIG. 11.

FIG. 13 is a cross-section view of the inlet portion of the air flowtube taken along the viewing plane 13--13 of FIG. 11.

FIG. 14 is a cross-section view of the nozzle portion of the air flowtube taken along the viewing plane 14--14 of FIG. 11.

FIG. 15 is an alternate embodiment of the exit mouth of the sensor airflow tube.

FIG. 16 is another alternate embodiment of the exit mouth of the sensorair flow tube.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring first to FIGS. 1 through 5, the improved sensor 10 includes asensing portion 11 with a sensor block 13 and a light-scatter sensingcavity 15. The cavity 15 has a reflecting mirror 17, e.g., an ellipticalmirror, and a detector 19 mounted to receive scattered light 24scattered by a particle 23 and reflected by the mirror 17. Referringalso to FIGS. 6-11, an air flow tube 25 is mounted to the block 13,extends along a flow axis 27 and includes a slightly tapered inlet end29 for attaching one end of a length of hose 31. A probe (not shown) isattached to the other end of the hose 31 and air with entrainedparticles flows through the hose 31 and the flow tube 25 and is expelledat the exit mouth 33.

The sensing portion 11 also includes a laser diode assembly 35 whichprovides a very thin, substantially cylindrical beam of light 21 along abeam axis 37. The mirror axis 39, the beam long axis 37 and the flowaxis 27 are preferably orthogonal; that is, they are mutuallyperpendicular.

As best seen in FIGS. 2 and 3, the sensor 10 also has a housing 22 witha blower portion 41 including a blower cavity 43 in which is mounted anair blower 45 with an electric drive motor 47 attached thereto. Suchmotor may be AC or DC, the latter for easier battery-poweredportability. Battery power is provided to the motor 47 through the leads49. A printed circuit board 51 provides closed loop feedback control forspeed regulation at a set point selected using the flow sensor 53. Speedcontrol is by pulse width modulation.

The bottom cover 55 of the cavity 43 has an exhaust filter 57 throughwhich must pass all air flowing through the sensor 10. The filter 57helps avoid contaminating the environment (which may be a "clean room")with particulates sloughed from the blower 45 itself and/or drawn inthrough the air flow tube 25.

Before describing particular details of a preferred air flow tube 25, anoverview comment will be helpful. In general, the air flow tube 25 has arelatively large cross-sectional area 58 at its inlet end 29. Viewedfrom left to right in FIG. 11, such cross-sectional area 58 remainssubstantially constant along a first segment 59 of the tube 25 and thengradually diminishes at a second segment 61. Its smallestcross-sectional area is at the exit mouth 33. The following moredetailed description is based upon that general configuration.

Referring particularly now to FIGS. 6-11, the air flow tube 25 includesan inlet portion 63 (which is generally T-shaped in cross-section) and anozzle portion 65, the upstream end 67 of which is snugly fitted into apocket 69 formed in the downstream end 71 of the portion 63.Particularly, the upstream end 67 abuts an annular shoulder 73, theinward edge 75 of which defines an area 76 having substantially the samesize and shape as the area 76 defined by the inner surface 77 of theupstream end 67. Preferably, those areas 76 are circular and ofsubstantially the same diameter.

The area of the passage 79 in the inlet portion 63 gradually increasesfrom the shoulder 73 leftward as viewed in FIG. 11. The passage 79attains maximum area 58 at a location 81 between the inlet end 29 andthe segment 61. In a highly preferred arrangement, the segment 61resembles a truncated cone.

Certain features of the preferred air flow tube 25 will now be describedusing a few "sections," i.e., profiles like cross-sectional views asaspects of the tube 25 would appear if cut through by an intersectingplane. Referring now to FIGS. 11-14, the inlet portion 63 has a firstinlet section 83 of minimum cross-sectional area 76. That is, thecross-sectional area 76 of the inlet section 83 is less than thecomparable area at any other section along the length of the inletportion 63. And the inlet portion 63 has a second inlet section 85having a cross-sectional area 58 greater than that of the first inletsection 83. This relationship is apparent from a comparison of FIGS. 12and 13.

The nozzle portion 65 has a first nozzle section 87 of maximumcross-sectional area. The area of section 87 is as great or greater thanthe comparable area of any other section along the length of the nozzleportion 65. And a visual comparison of FIGS. 13 and 14 demonstrates thatthe cross-sectional area of the first inlet section 83 is no less than(and is preferable about equal to) the cross-sectional area of the firstnozzle section 87.

In one highly preferred embodiment, the interior passage 79 of the airflow tube 25 is circular in cross-section along most of its length,i.e., up to that part of the nozzle portion 65 at which such portion 65necks down and fans out to define the exit mouth 33. As best seen inFIG. 9, a preferred exit mouth 33 has an area 62 and first and secondside edges 89a and 89b, respectively, which are generally parallel toone another. The mouth 33 has rounded end edges 91 and the resultingmouth shape resembles that of a race track.

While the exit mouth 33 shown in FIG. 9 is preferred, there are otherpossibilities. For example, FIG. 15 shows an ovoid mouth 33 and FIG. 16shows a somewhat bow-shaped mouth 33. However, it is preferred that themaximum width "W" of any mouth 33 be about equal to or at least notappreciably greater than the diameter of the beam of light 21.Maintaining that width relationship helps assure that all orsubstantially all of the air-entrained particles 23 flowing out of theexit mouth 33 pass through the beam of light 21. On the other hand, anelongate exit mouth 33 helps assure reduced pressure drop as comparedto, say, a circular exit mouth having a diameter about equal to thediameter of the light beam 21.

And there are also other relationships that characterize the preferredembodiment. Referring to FIGS. 5 and 9, the exit mouth 33 has a majoraxis 93 and a minor axis 95 generally normal to one another. The mouth33 is oriented so that the major axis 93 is generally parallel to andspaced slightly from the beam long axis 37. Considered another way, theexit mouth 33 is elongate in a direction substantially parallel to thebeam long axis 37.

Referring again to FIGS. 1, 2 and 3, the cavities 15 and 43 areseparated by a wall 97 having a wall aperture 99. The blower 45 ismounted to an annular plate 101 which has an opening 102 through it andthe blower 45 itself has a side air entry port 103 through which theblower 45 draws air for expulsion through the exhaust port 105. In apreferred arrangement, the wall aperture 99, the plate opening 102 andthe air entry port 103 are in registry with the flow axis 27 and, mostpreferably, are generally concentric with such axis 27.

From the foregoing, it is to be appreciated that all of the air passingthrough the blower 45 is drawn from and first through the air flow tube25. Blower-generated contaminants, e.g., paint chips, metal "fines" andthe like, do not contaminate the air stream and do not enter the sensingcavity 15 where they might impair the accuracy of the sensor 10. To putit another way, the blower 45 is used "inside out" with respect to itsconventional use mode.

The pressure adjacent to the blower air entry port 103, i.e., on theintake side of the cage-like rotor. The air entry port 103 is in flowcommunication with the exit mouth 33. The blower 45 thereby provides, inthe form of a pressure differential between the inlet end 29 and theexit mouth 33 of the air flow tube 25, the "motive force" moving airthrough the tube 25.

Referring again to the FIGURES, in operation, the blower 45 is energizedand air (usually with at least some particles 23 entrained therein) isdrawn into the inlet end 29 of the air flow tube 25. The air-propelledparticles 23 are expelled from the exit mouth 33 and "fly" through thelaser beam of light 21. Beam of light 21 reflected by such particles 23is received by the mirror 17 and reflected to a detector 19 forelectronic analysis.

Air and entrained particles 23 continues to flow through the aperture99, the opening 102 and the port 103 in the blower 45 and is dischargedby the blower 45 through its exhaust port 105. Such air is urged througha coarse filter disc 107 which helps "smooth" air flow from turbulent tolaminar flow. A major portion of the air is then simply exhaustedthrough the openings and "free-flows" through the blower cavity 43 andexhaust filter 57 at the bottom of such cavity.

A relatively small percentage of the air from the blower exhaust port105 enters a barbed fitting 109 and flows along the tubing 111 andthrough the flow sensor 53. From the sensor 53, such air flows along thetubing 113 and back into the blower cavity 43 from which it, too,free-flows out the exhaust filter 57.

It has been found that the new sensor 10 exhibits no greater pressuredrop than 7-10 inches of water along the air flow tube and, moretypically, such pressure drop is about 3 inches of water. This is astartling contrast to the pressure drop of 25-70 inches of waterexhibited by prior art sensors.

While the principles of the invention have been described in connectionwith a few preferred embodiments, it is to be understood clearly thatsuch embodiments are by way of example and are not limiting.

We claim:
 1. In a particle sensor having a light beam with a beam longaxis and further having an air flow tube defining a flow passage havinga pressure drop therealong, such tube having (a) an inlet end with across-sectional area, and (b) a particle exit mouth having across-sectional area, the improvement wherein:the cross-sectional areaof the flow passage at the inlet end is greater than the cross-sectionalarea of the exit mouth; the exit mouth is elongate in a directionsubstantially parallel to the beam long axis; the exit mouth has alength measured generally parallel to the beam long axis; the air flowtube extends along an air flow path; air flow through the air flow tubeis turbulent; the sensor has a speed-adjustable centrifugal blower whichhas an air entry port; the sensor has a sensor cavity and a blowercavity which are connected by a passage-like aperture; and the apertureand the air entry port are along the flow path.
 2. A particle sensorusing scattered light for analyzing airborne particles entrained in airdrawn from an environment into the particle sensor, such sensorincluding:a variable-speed centrifugal blower having a motor and anexhaust port; a housing surrounding the blower; a particle detectionsystem including a sensing portion connected to the housing and having alight-scatter sensing cavity; an air flow tube and a low-pressure-dropnozzle in air flow communication with the blower, the air flow tube andnozzle extending from the environment into the sensing cavity; a circuitconnected to the motor for providing a speed-controlling variablevoltage to the motor; a flow sensor for providing a speed-affectingsignal to the circuit; an exhaust filter interposed between the exhaustport and the environment;and wherein: the particle detection systemfurther includes (a) a laser light source, (b) a device directing lightscattered by a particle entering the cavity through the air flow tubeand the nozzle, and (c) a detector for receiving light directed by thedevice; and the air flowing in the sensing cavity through the air flowtube is drawn through such sensing cavity by the centrifugal blower,flows at a flow rate dependent upon the speed of the blower motor and isdischarged from the sensor through the exhaust filter,whereby (a) therate at which air is drawn into the sensing cavity may be varied, and(b) particulate contaminants are substantially prevented from beingexhausted to the environment.
 3. The particle sensor of claim 2wherein:the air flow tube extends along a linear flow axis; thecentrifugal blower has a side air inlet port; and the air inlet port iscoaxial with the flow axis.
 4. The particle sensor of claim 2wherein:the circuit is of the closed loop feedback type; and a tubeextends from the blower exhaust port to the flow sensor for providing afeedback signal to the circuit.